Due to a recent trend of environmental consciousness, controls on lead included in solder alloys will be imposed in the field of electronic packaging, and thus, establishment of a lead-free packaging a technique, i.e., a technique to join electronic elements with a lead-free material is an urgent necessity. The lead-free packaging technique includes packaging using mainly a lead-free solder or a lead-free conductive adhesive. Conductive adhesives have been noted particularly in the technique since they are expected to provide merits such as joint flexibility, lower packaging temperatures, no need of organic solvents, and no need of washes.
A conventional conductive adhesive is based on, e.g., an epoxy resin-based binder resin in which a conductive filler of a metal powder such as silver is dispersed. When an electronic element and a substrate electrode are connected through the conductive adhesive, the binder resin contacts the conductive filler with other conductive filler, the element electrode, and with the substrate electrode so as to provide electrical connection while the electronic element and the substrate electrode are adhered for mechanical connection. Since the electronic element and the circuit substrate are connected at the joints with a resin component, it deforms flexibly corresponding to deformation caused by heat and external force. Therefore, it has less cracks when compared to solder connection with an alloy. Moreover, a typical conductive adhesive has a joint temperature as low as 150° C. while the joint temperature of an alloy is about 240° C., electronic elements with reduced heat resistance can be used. Furthermore, the energy requirement for the production can be reduced.
Therefore, a conductive adhesive is expected as an alternative to solder, since it has excellent performance over solder connection.
However, a conventional conductive adhesive as a replacement for solder cannot provide a connection strength corresponding to that of solder. Another problem is that such an adhesive is more expensive when compared to solder as package materials of electronic elements.
The following description is on connection strength. A conductive adhesive adheres to an electronic element and to a substrate electrode since, for example, an epoxy-based binder resin adheres to the element and the substrate electrode. Among resin materials, the epoxy-based binder resin has an especially strong adhesion with metals, and the resin has excellent mechanical strength after hardening. Therefore, it has been used for adhesives for many elements. However, unlike solder, since the joint is not provided by alloy, the adhesives cannot provide the connection strength of solder against external force applied to the connected part due to bending of the substrate and impacts. The main factors are as follows.
The epoxy resin as a binder component for a conventional conductive adhesive has strong adherence to metal of a substrate electrode. However, it has a high elastic modulus and less flexibility. When a substrate comprising such a resin is subjected to bending deformation, stress is concentrated at the joint interface between the electronic element and the conductive resin. When the stress exceeds the strength of connection between the electronic element and the conductive resin, peeling may occur at the interface. Such an adhesive resin cannot correspond to deformation caused by bending, vibration and impact generated in the substrate at the connected part of the electronic element and the circuit substrate.
Concerning the flexibility of conductive resin, JP-A-3-21868 for utility model suggests an elastic conductive adhesive containing an elastic adhesive as a binder resin component.
This conductive resin described in JP-A-3-21868 has more improved flexibility when compared to the other examples of epoxy resins. However, it provides less conductivity when compared to an epoxy resin that has conductance provided by contraction of the hardened resin. And the conductive filler is of a spherical shape, scale shape, or a mixture thereof. Therefore, it is difficult to decrease the resistance of the conductive resin to the level of the resistance of an epoxy-based resin.
In a conventional technique, a typical conductive adhesive contains a conductive filler of about 85 vol. %. A conductive filler such as silver has a specific weight of about 10 while the binder resin has a specific weight of about 1.1. This substantially decreases the mechanical connection or the connection strength at the connected part to a half, i.e., the area that the binder resin contacts with the element electrode and the substrate electrode is halved. As a result, the connection strength of the adhesive containing filler will be lowered when compared to a case using a binder resin alone.
As mentioned above, a conventional conductive adhesive has a high elastic modulus that leads to problems in adhesion strength with either an electronic element or with a circuit substrate, and also in connection reliability. Also, the conductive adhesive has problems concerning joint interface with the electronic element and with the circuit substrate.
A conventional conductive resin contains a conductive filler such as silver powder in as much as about 85 vol. %. Since the conductive filler accounts for about 70% or 80% of the cost for the conductive adhesive, it is substantially impossible to reduce the cost.
Conventional conductive adhesives have a merit from the aspect of flexibility over solder but the connection strength with respect to dynamic deformations such as substrate bending, vibration, and impact is insufficient. Moreover, the conductive adhesive raises the manufacturing cost. These demerits inhibit the conductive adhesives from replacing the commonly used solder for connection.